(a) Field of the Invention
The present invention relates to a microscope objective lens system.
(b) Description of the Prior Art
It is necessary that various aberrations be sufficiently well-corrected in a microscope objective lens system. But, at the same time, it is necessary to satisfy the requirements which limit the freedom of the correction of aberrations. For example, in order to obtain high resolution power, it is necessary for the numerical aperture (NA) to be large. In order to avoid a damaging contact between an object and an objective lens system, it is necessary for the working distance (WD) to be large. In order to set the various kinds of objective lens systems on the same revolver and to change them for use, it is necessary for a total length of the lens system to be within predetermined limits. Moreover, in order to keep the optical tube length constant, it is necessary for the distance between the rear focal point of an objective lens system and the image surface to be constant independently of magnification.
It is very difficult to correct various aberrations sufficiently given these various limitations plus the limitation that the objective lens system is a magnifying system.
In order to overcome these difficulties, the conventional microscope objective lens system cannot but be an extremely complicated optical system which comprises a number of lenses having strong curvatures.
In order to simplify construction, there have been developed microscope objective lens systems in which aspherical lenses or graded refractive index lenses (GRIN lenses) are introduced. The examples where a GRIN lens is applied to a microscope objective lens system are disclosed in Japanese Published Examined Patent Application Nos. 28057/72 and 37405/82. The former only discloses the use of a GRIN lens in the image-forming system of a microscope, but does not disclose anything related to the correction of aberrations. On the other hand, in the latter, a GRIN lens whose refractive index is expressed by the exponential function as described below is arranged, in the optical system of a microscope, at the position where the height of the axial marginal ray is smaller than that of the off-axial ray, so that off-axial aberrations are corrected. EQU n=n.sub.0 e.sup.kr
where n.sub.0 represents the refractive index at the center of the lens, k is the parameter representing the gradient of the refractive index and r represents the radial distance from the optical axis.
In the case that the refractive index varies in accordance with the above-mentioned function, it varies extraordinarily where r is large. Therefore, when the above-mentioned GRIN lens is arranged at the above-mentioned position, it will be possible to correct only off-axial aberrations without any influence on axial aberration.
In a usual microscope objective lens system, however, when NA becomes large, axial light bundle and off-axial light bundle propagate almost overlappingly so that the position where both the bundles separate from each other is quite close to the image-forming position. In the conventional construction, therefore, it is necessary to set a GRIN lens separately and behind an objective lens. However, the position through which a ray passes is different in each objective lens system, so that when various kinds of objective lens systems are changed while the GRIN lens is fixed, it will be impossible to always well-correct aberrations.